Red light debut for exotic 'metamaterial'

The race to build an exotic material with a negative refractive index for visible light has been won by a team of researchers in Germany. The demonstration could open the door to a new generation of optical devices such as superlenses able to see details finer then the wavelength of visible light.

It may also, ultimately, lead to further breakthroughs in “invisibility cloaks” which could hide objects from the human eye.

Light waves consist of alternating electric and magnetic fields that interact with materials as they travel, or propagate. This interaction determines a property of the material called its refractive index, which is a measure of the behaviour of light as it passes through the material. The refractive index describes the way the light waves bend when they enter and leave the material and the speed at which they propagate.

The refractive index of normal materials is always positive – 1.0003 in air, about 1.5 in ordinary glass, 2.1 in zircon, and 2.4 in diamond. In the mid-1990s, however, John Pendry of Imperial College London, UK, realised that it was possible to construct artificial materials in which the refractive index could be negative.

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Scaling down

The trick is to assemble an array of electronic components that resonate with the electric and magnetic fields of the light waves as they pass through. These materials are unlike any conventional substance, hence the name “metamaterial”. Pendry suggested that an array of coils and wires much smaller than the wavelength of light would do the trick and first demonstrated the idea for radio waves with a frequency between 15 and 20 megahertz.

Later experiments extended the technique to shorter wavelengths, first into the microwave region and later the infrared. Now Gunnar Dolling at the University of Karlsruhe in Germany, and colleagues, have demonstrated the effect at 780 nanometres – the long-wavelength end of the red spectrum – by scaling down a structure he had developed for infrared wavelengths.

Dolling’s metamaterial is made by depositing a layer of silver on a glass sheet, covering this with a thin layer of nonconducting magnesium fluoride, followed by another silver layer, forming a sandwich 100 nm thick. Dolling then etched an array of square holes through the sandwich to create a grid, similar to a wire mesh.

Bending backwards

Dolling determined the refractive index of the material by measuring the “phase velocity” of light as it passed through. His measurements show the structure has a negative refractive index of -0.6 for light with a wavelength of 780 nm.

This value drops to zero at 760 nm and 800 nm, and becomes positive at longer and shorter wavelengths. Previously, the shortest wavelength at which a negative refractive index had been demonstrated was 1400 nm.

The team has not yet observed some of the other exotic effects possible with a negative refractive index, such as the ability to bend light backwards. However, simulations show that negative-index lenses should produce exotic effects over a limited range of wavelengths. For now, Dolling is concentrating on studying the new effects rather than attempting to build devices such as superlenses. These applications are still a long way off, he told New Scientist.